1. Field of the Invention
The present invention relates to a zoom lens system, and more specifically, to a zoom lens system for use in a lens shutter camera incorporating a zoom lens system.
2. Description of the Prior Art
Presently, a compacter, lower-cost taking lens system is demanded to realize a compacter, lower-cost lens shutter camera incorporating a zoom lens system. On the other hand, a zoom lens system having a large zoom ratio is demanded.
In order to reduce the size of a zoom lens system including movements of lens units during zooming while maintaining a sufficient zoom ratio, it is necessary to increase the refractive power of each unit.
However, increasing the refractive power while maintaining a sufficient performance results in an increase in the number of lens elements. On the other hand, in order to reduce the cost, it is effective to reduce the number of lens elements.
As described above, reducing the size and reducing the cost while maintaining a sufficient zoom ratio conflict with each other.
As a zoom lens system which is intended to realize a smaller size and a higher zoom ratio while maintaining a sufficient optical performance, zoom lens systems have been proposed (in Japanese Laid-open Patent Application No. S64-72114, Japanese Laid-open Patent Application No. H2-63007, Japanese Laid-open Patent Application No. H2-181715 and Japanese Laid-open Patent Application No. H2-201409) which comprise from the object side a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and a third lens unit having a negative refractive power and where zooming is performed by moving each lens unit.
These zoom lens systems, however, do not meet the present demand for a compacter, lower-cost zoom lens system. That is, in the zoom lens systems, it is difficult to reduce the number of lens elements and hold down the cost while maintaining a sufficient optical performance, and the size is not sufficiently reduced.
Generally, lens shutter cameras incorporating a zoom lens system employ a front focusing method where a first lens unit arranged on the most object side is moved during focusing. This method, however, has a defect that it is necessary to increase the effective diameter (front lens diameter) of the first lens unit in order to secure a sufficient peripheral illumination in photographing a subject located close to the camera. As a result, the size of the entire camera system and a load on a motor for moving the lens unit increase.
Another method has been suggested where focusing is performed by means of a lens unit other than the first lens unit. For example, Japanese Laid-open Patent Application No. H2-201409 discloses a zoom lens system comprising from the object side a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and a third lens unit having a negative refractive power. Focusing is performed by moving the third lens unit.
However, this method where focusing is performed by mean of the third lens unit has a defect that it is necessary to increase the effective diameter of the third lens unit similarly to the previously-mentioned method where focusing is performed by mean of the first lens unit. In this case, the size of the entire camera system also increases.